This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Methanogens produce approximately one billion tonnes of methane every year, which is a clean source of renewable energy, but also poses environmental hazards as a potent greenhouse gas. Methyl CoM reductase (MCR) catalyzes the terminal step of methanogenesis in which coenzyme B(CoB) and methyl coenzyme M are converted to methane and CoMS-SCoB (a heterodisulfide). The enzyme contains a Ni porphinoid F430 active site, which is active in the Ni(I) (d9) oxidation state. Extensive spectroscopic and theoretical studies have been performed on MCR, which indicate that the catalytic cycle involves an organometallic NiIII-Me species. However, neither the Ni(I), nor the Ni(III)-Me forms have been crystallographically characterized. Although, crystal structures of the inactive Ni(II) forms have been solved, elucidating the geometric and electronic structures of the Ni(I) and Ni(III) forms is key in understanding the catalytic mechanism of methane formation.